1. Field of the Invention
The present invention relates to receiving technologies, and it particularly relates to method and apparatus for receiving signals by a plurality of antennas.
2. Description of the Related Art
In wireless communication, it is generally desired that the limited frequency resources be used effectively. Adaptive array antenna technology is one of the technologies that effectively utilize the frequency resources. In adaptive array antenna technology, the amplitude and phase of signals transmitted from and received by a plurality of antennas, respectively, are so controlled as to form a directional pattern of the antenna. In other words, the apparatuses provided with adaptive array antennas change respectively the amplitude and phase of signals received by the plurality of antennas, add up respectively a plurality of the thus changed received signals, and receive the signals equivalent to the signals received by the antenna having the directional pattern corresponding to the variation in said amplitude and phase (hereinafter referred to as “weight”). In addition, signals are transmitted in a directional pattern of the antenna corresponding to the weight.
One example of processings to compute the weights in adaptive array antenna technology is a processing based on the MMSE (Minimum Mean Square Error) method. In the MMSE method, the Wiener solution is known to be the condition for an optimum weight value. Also known is a recurrence formula whose amount of calculation is smaller than directly solving the Wiener solution. For such recurrence formula, adaptive algorithms, such as RLS (Recursive Least Squares) algorithm and LMS (Least Mean Squares) algorithm, are used. The RLS algorithm generally converges faster, but the computation therefor is complicated, thus requiring high-speed arithmetic circuits or large-scale arithmetic circuits. Though the LMS algorithm can be realized with arithmetic circuits which are simpler than those for the RLS algorithm, the convergence speed thereof is low (See Reference (1) in the following Related Art List, for instance).
Related Art List
(1) Japanese Patent Application Laid-Open No. 2002-26788.
When the adaptive array antennas are used for the radio mobile station, it is desirable that the arithmetic circuit be of a small size. Thus, the use of the LMS algorithm is appropriate for the updating of the weighting factors. However, the LMS algorithm generally converges slowly. Thus, if the received signals to be combined or synthesized are to be delayed until they converge, the processing delay will be accumulated. Accordingly, real-time processed applications, such as those for video conference, where permissible delay time is constrained to a certain time period may not be used. On the other hand, if the weighting factors at a stage where the LMS algorithm has not converged are used to carry out the receiving processing, then the receiving characteristics will generally be deteriorated.